Set up an extremely lightweight set of tests for the user infra implementation

src/infra/db_test.py

@@ -0,0 +1,8 @@
+from src.infra import Base
+from sqlalchemy import Engine, create_engine
+
+
+def mock_db() -> Engine:
+    engine = create_engine('sqlite:///:memory:', echo=True)
+    Base.metadata.create_all(engine)
+    return engine